EP0949800B1

EP0949800B1 - Image reading apparatus and control method therefor

Info

Publication number: EP0949800B1
Application number: EP99302741A
Authority: EP
Inventors: Hisao C/O Canon Kabushiki Kaisha Terajima; Yasuyuki C/O Canon Kabushiki Kaisha Shinada; Takashi C/O Canon Kabushiki Kaisha Ono
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1998-04-10
Filing date: 1999-04-08
Publication date: 2006-07-05
Anticipated expiration: 2019-04-08
Also published as: DE69932200D1; JP3459770B2; US6785026B1; JPH11298673A; EP0949800A2; CN1236257A; DE69932200T2; CN1156141C; EP0949800A3

Description

The present invention relates to an image reading apparatus and control method therefor suitably used for, e.g., a colour scanner or colour facsimile apparatus having an image reading unit which has three, R, G, and B (Red, Green, and Blue) light sources and reads an original image by sequentially turning on the light sources.
A colour scanner orcolour facsimile apparatus having a reading unit capable of reading a colour image by switching the emission colour of light sources has conventionally been known. This apparatus uses three, red, green, and blue (to be referred to as R, G, and B) LEDs as a light source, and a line image sensor as a reading unit. In this image sensor, light of each colour emitted from the LED illuminates a portion of an original including an image, and the light reflected by the original surface is incident on a sensor IC via a lens array.
The line image sensor constituted by the sensor IC has a photoelectric conversion element such as a photodiode and a capacitor for each pixel, and converts incident light into a current to accumulate the current as charges in the capacitor. The accumulated charges are sequentially converted into a voltage and output from the sensor. The voltage output is converted into digital data by an A/D converter, and the digital data is subjected to various image processes to generate a scanner or facsimile output. Colour read operation in a general colour scanner having light sources of three colours is as follows. That is, the red LED is turned on to read one line in the main scanning direction, thereby obtaining the red component of one line of acolour image. The green LED is turned on to obtain the green component of one line. The blue LED is turned on to obtain the blue component of one line.
In the colour scanner of this type, an original is conveyed in the subscanning direction during an image read for one line by the LEDs of three colours. After one line is read by the LEDs of three colours, the original has been conveyed by one line. That is, the original read operation and the convey operation in the subscanning direction are simultaneously performed. These operations are repeated a plurality of number of times to read a colour image of one original page. This read control method will be called "scan and read".
In this reading system, since the LED light sources of the respective colours have different light quantities, a voltage output from the image sensor may exceed a predetermined A/D convertible range of the A/D converter. An output voltage from the image sensor must be limited to fall within the A/D convertible range of the A/D converter by adjusting the duty ratio of the ON time of the LED of each colour in one period of time (to be simply referred to as period) of a line sync signal.
The LEDs are turned on in synchronism with a line sync signal but turned off at different timings in units of colours (in units of LEDs). More specifically, when the light emission quantity of LED is large, the LED is turned off soon; when the light emission quantity is small, the LED is turned off late. The OFF timing is automatically determined by the CPU for controlling the apparatus. While the CPU turns on these LEDs, an original is actually read. In this read control, an original is conveyed in the subscanning direction during read of the original. Since the read period is different between the respective colours the width of the read range in the subscanning direction is different between the respective colors.
This will be shown in Fig. 8. In Fig. 8, the ON periods (high-level portions of rectangular waves) of LEDs of the respective colours are longer in the order from R, G, and B. The light-receiving unit of the image sensor has an opening with a width corresponding to one line in the subscanning direction. The image sensor converts the light quantity reflected by an original surface facing this opening into charges, and accumulates the charges. An original surface facing the sensor opening from the ON timing to the OFF timing of the light source is an actual read range. As is apparent from Fig. 15, the width of the read range in the subscanning direction is longer in the order from R, G, and B.
In reading a monochrome image by the colour facsimile apparatus, i.e., in reading a facsimile transmission image, all the light sources of three colours are simultaneously turned on as a white light source to read an image. Alternatively, an image is read using only a light source of one colour, e . g . , green out of light sources of three colours.
However, as described above, since the width of the read range in the subscanning direction is different between the respective colours the resolution in the subscanning direction differs between the respective colours. As a result, read data of an originally black thin horizontal line does not indicate black, resulting in poor colour reproducibility in reading a halftone original.
In the prior art, when the light sources of three colours are simultaneously turned on to read a monochrome image, the power is greatly consumed. The light emission quantity of the light source increases to require a heat dissipation structure, resulting in a large outer shape of the reading unit. Since the three light sources vary in light quantity, they do not function as a complete white light source. If an original to be read is a colour original, a colour close to one having a large light quantity has a low read density, and the original density cannot be accurately expressed. This problem becomes more serious in reading an original in a pseudo halftone mode.
To the contrary, when a monochrome original is read by turning on only a light source of one colour (monochrome mode), if monochrome originals are frequently read, the light source of that colour degrades sooner than the remaining light sources. The degradation of the light source.must be finely corrected in reading a colour original (colour mode). This problem becomes notable when an LED is used as a light source. When a colour original is read in the monochrome mode, the same colour as the light source cannot be read.
The present invention has been made in consideration of these problems. In the embodiments described hereinbelow, it is possible to attain the same resolution in the subscanning direction for the respective colours in reading an image and improve the colour reproducibility of a black thin horizontal line or halftone original even if variations in light quantities of light sources of a plurality of colours are adjusted in a reading system of reading an original image while scanning.
In the embodiment of the present invention described hereinbelow, it is possible to read a high-quality image without any dropout colour with smaller power consumption and eliminate variations in degradation of light sources when a monochrome image is read.
The image reading apparatus of the present invention is an apparatus of the kind such as known from United States Patent No. 5450215 which is adapted to read an original image by using a plurality of emission colours of light which are sequentially generated by plural light sources in accordance with an emission control signal and detecting light of each colour from the original image by an image sensor.
The method of controlling such apparatus, as known from US 5450215, includes a step of supplying the emission control signal and detecting light of each colour from the original image by an image sensor, which method includes a step of:

supplying the emission control signal so as to change the emission colour of light generated by said plural light sources every predetermined charge accumulation period of the image sensor.

An image reading apparatus according to the present invention, as also a method of controlling the same, are set out in the appended claims.
Each of the following embodiments provides an image reading apparatus which reads an original image by sequentially irradiating an original image with a plurality of emission colours and detecting light of each colour from the original image by an image sensor which apparatus comprises a light source capable of sequentially generating the plurality of emission colours, and control means for setting a flickering duty ratio of the light source for each emission colour, when the original image is read, changing the emission colour every predetermined charge accumulation period of the image sensor, and flickering the light source in accordance with the duty ratio set for each emission colour.
The control means may set a flickering duty ratio of another emission colour using, as a reference, a total ON time of one of the plurality of emission colours in a predetermined charge accumulation period of the image sensor, when the original image is read, change the emission colour every charge accumulation period, and flicker the light source with another emission colour in accordance with the duty ratio set for each emission colour.
At this time, for example, the control means calculates the flickering duty ratio of the light source for each emission colour calculates a product of the calculated duty ratio and the predetermined charge accumulation period of the image sensor to calculate the total ON time for each colour and selects a product of an emission colour having the largest calculated value as the reference total ON time.
In any of the above apparatus arrangements, when a reference white background is irradiated by flickering the light source with one of the plurality of emission colours, and an output value from the image sensor is smaller than a predetermined value and comes nearest to the predetermined value, the control means sets a flickering duty ratio at this time as an optimum duty ratio for the emission colour, the control means repeating the duty ratio setting operation for each of the plurality of emission colours.
Further, in a monochrome mode, the control means sequentially changes the emission colors during the charge accumulation period, and forms a pseudo white light source by turning on the light source in accordance with the light emission time set for each emission colour.
The present invention provides an image reading apparatus which reads an original image by detecting, by an image sensor, light from the original image irradiated by a light source comprises control means for controlling a single light source for irradiating the original image to flicker a plurality of number of times every predetermined charge accumulation time of the image sensor when the original image is read.
The control means adjusts a flickering duty ratio when the light source flickers.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof wherein:

Fig. 1 is a block diagram showing an example of the arrangement of acolour scanner according to the first embodiment of the present invention;
Fig. 2 is a view for explaining the relationship between the ON state of each LED and the original read position according to the first embodiment of the present invention;
Fig. 3 is a flow chart showing interrupt processing according to the first embodiment of the present invention;
Fig. 4 is a flow chart showing a process of determining the duty ratio of the LED in the first embodiment;
Fig. 5 is a block diagram showing an example of the arrangement of a colour scanner according to the second embodiment of the present invention;
Fig. 6 is a view for explaining the relationship between the ON state of each LED and the original read position according to the second embodiment of the present invention;
Fig. 7 is a flow chart showing interrupt processing according to the second embodiment of the present invention; and
Fig. 8 is a view for explaining the relationship between the ON state of each LED and the original read position in the prior art.

Embodiments of the present invention will be described below with reference to the accompanying drawings.

First Embodiment

Fig. 1 best shows the feature of an image reading apparatus according to the present invention by exemplifying the arrangement of a colour scanner. In Fig. 1, reference numeral 101 denotes a sensor unit constituted by integrating an image sensor such as a CCD (Charge Coupled Device) and a light source such as an LED (Light Emitting Diode); and 102, a read control unit for controlling an original reading unit and performing image processing of read data.
Reference numeral 103 denotes a CPU for controlling the whole system; 104, a RAM used as a work area of the CPU 103 and an image data memory area; 105, a ROM for storing control programs of the CPU 103; and 106, a DMA controller for performing DMA (Direct Memory Access) transfer between the read control unit 102 and RAM 104.
The read control unit 102, CPU 103, RAM 104, ROM 105, and DMA controller 106 are connected to each other via a system bus 107. The CPU 103 accesses the blocks connected to the system bus 107 in accordance with a control program in the ROM 105.
The internal arrangement of the sensor unit 101 will be described. Reference numerals 108, 109, and 110 denote LED light sources for emitting R, G, and B light beams; 111, 112, and 113, switches which control the ON/OFF states of the R, G, and B LED light sources 108, 109, and 110, are formed from transistors in the first embodiment, and turn on the LEDs while the collector-to-emitter path is ON; and 114, 115, and 116, limiting resistors for the LED light sources 108, 109, and 110.
Reference numeral 117 denotes a conductor for dispersing light input from the LED light sources 108, 109, and 110 in the main scanning direction and uniformly illuminating a read line on an original 120; and 118, a lens array. Light reflected by the original 120 is input via the lens array 118 to a sensor IC 119 where the light is converted into an electrical signal, i.e., a voltage proportional to the light reflected by the original 120. When the sensor IC 119 receives a line sync signal SYNC from a timing control unit 129 in the read control unit 102, it initializes an internal pixel counter and sequentially outputs converted voltages in light-receiving elements aligned in the main scanning direction.
The internal arrangement of the read control unit 102 will be described. Reference numeral 121 denotes an A/D converter which converts an analog output voltage from the sensor IC 119 into digital multivalued image data, and converts a voltage into digital data of about 256 gray levels per pixel; and 122, a correction circuit for correcting variations in sensitivity of the sensor 119, variations in light quantities of the LED light sources 108, 109, and 110, and nonlinearity of the sensor 119 with respect to an incident light quantity by performing digital calculation or conversion with reference to a table for a digital output from the A/D converter 121, and for outputting the obtained data.
Reference numeral 123 denotes a DMA interface for receiving image data corrected by the correction circuit 122 and outputting the input data to an internal bus 124 in accordance with timing control of the DMA controller 106. The image data output to the internal bus 124 is written in the RAM 104 via the system bus 107.
Reference numeral 125 denotes a colour register for storing a light source colour designation signal for designating a light source to be turned on. The output of the colourregister 125 is connected to the switches 111, 112, and 113 in the sensor unit 101 via AND gates 130, 131, and 132. When one of the three outputs of the colour register 125 changes to high level, a corresponding transistor is turned on to turn on a corresponding light source. The colour register 125 receives data in a register 126 in synchronism with a sync signal SYNC from the timing control unit 129. In other words, the colour register 125 updates the light source colour designation signal as an output from the colour register 125 in synchronism with the sync signal SYNC.
Reference numeral 127 denotes a PWM (Pulse Width Modulation) waveform generator which generates a periodic rectangular wave, incorporates a register for storing a flickering duty ratio, and generates a waveform having a predetermined duty ratio of high and low levels in accordance with the value of the internal register. This internal register receives data in a register 128 in synchronism with the sync signal SYNC from the timing control unit 129. The duty ratio is determined and stored in the register in steps S4 to S7 in Fig. 2 (to be described later). The duty ratio of the PWM waveform generator 127 is updated in synchronism with the sync signal SYNC.
The output of the PWM waveform generator 127 is connected to the switches 111, 112, and 113 in the sensor unit 101 via the AND gates 130, 131, and 132. By this connection, any one of the AND gates 130, 131, and 132 selected by a light source colour designation signal in the colour register 125 outputs a waveform generated in the PWM waveform generator 127. Then, any one of the LED light sources 108, 109, and 110 selected by the light source colour designation signal flickers with a duty ratio set in the internal register of the PWM waveform generator 127.
As described above, the timing control unit 129 generates the line sync signal SYNC. The sync signal SYNC is supplied to the CPU 103 in addition to the sensor IC 119, colour register 125, and PWM waveform generator 127. When the CPU 103 receives the sync signal SYNC, it shifts to interrupt processing and writes values in the registers 126 and 128 via the system and internal buses 107 and 124 in the interrupt routine. That is, the CPU 103 can reserve a light source colour designation signal and PWM duty signal for a next sync signal SYNC in the registers 126 and 128.
In the first embodiment, since the emission colour of the light source and the flickering duty ratio can be changed in synchronism with the sync signal SYNC, the light source can be controlled as shown in Fig. 2. That is, any R, G, and B light sources can be flickered at different optimum duty ratios set in the register 126 during one line sync period (i.e., a predetermined charge accumulation period of the image sensor included in the sensor unit 101). Even if the width of the original read range in the subscanning direction is the same for the respective light source colours the total light emission quantity by flickering during one charge accumulation period (to be simply referred to as an accumulation period) can be changed for each light source colour to appropriately control an output voltage from the image sensor 119.
While the LED light sources 108, 109, and 110 have different total light emission quantities during one line sync period, they can have the same resolution in the subscanning direction. An output voltage from the image sensor 119 can be limited to fall within the convertible range of the A/D converter 121, and the colour reproducibility of a thin horizontal line or halftone dot image on an original can be improved.
Setting of parameters in the registers 126 and 128 will be explained with reference to the flow chart in Fig. 3.
When the timing control unit 129 in Fig. 1 outputs a sync signal SYNC, the CPU 103 receives it and performs interrupt processing shown in the flow chart of Fig. 3. The interrupt processing has the following steps.
The CPU 103 determines the light source colour for the next line in step S1, and writes the determined colour information in the register 126 in step S2. As a result, the light source colour for a read which starts from the next sync signal SYNC is reserved. The CPU 103 determines the colour for the next output line in step S3. If the colour for the next output line is R, the CPU 103 advances to step S4; if the color is G, to step S5; and if the color is B, to step S6.
In steps S4, S5, and S6, the CPU 103 determines the ratios of the ON times of the LED light sources 108, 109, and 110 to one line sync period as PWM duty ratios. A method of determining the duty ratio will be described.
Fig.4 is a flow chart showing duty ratio determination processing in the first embodiment of the present invention which is executed by the CPU 103 for the LED light sources 108 to 110.
Steps S501 and S502 in Fig. 4: The CPU 103 determines whether an original is at a predetermined original read position (not shown) on the basis of an output value from an optical sensor (not shown). If NO in step S501 (the original is not at the original read position), the CPU 103 advances to step S503 (step S501). If YES in step S501 (the original is at the original read position), the CPU 103 retracts the original present at the original read position from this original read position to a predetermined retraction position by driving an original convey motor (not shown) (step S502).
Step S503: The CPU 103 sets an appropriate value as a provisional duty ratio in the register 126.
Steps S504 and S505: The CPU 103 turns on the selected LED in accordance with the duty ratio set in the register 126 to read a white reference plate set above the original read position (step S504) and detect a digital value output from the A/D converter 121 along with the read (step S505).
Step S507: If the value detected in step S504 is equal to or larger than FF, the current state is an overflow state in which the output value exceeds the A/D convertibility of the A/D converter 121. The CPU 103 sets in the register 126 a duty ratio smaller by predetermined value 1 than the duty ratio currently set in the register 126 (step S507) , and returns to step S504.
Step S506: If the value detected in step S504 is smaller than FF, the CPU 103 sets in the register 126 a duty ratio higher by predetermined value 2 than the duty ratio currently set in the register 126 (step S506), and returns to step S504. In this case, predetermined value 2 is preferably smaller than the absolute value of predetermined value 1.
Step S508 : If the value detected in step S504 is smaller than FF and is a maximum value at which the A/D converter 121 does not overflow, the duty ratio currently set in the register 126 is optimum for the selected LED, and the CPU 103 stores this duty ratio as an optimum duty ratio in the register 126.
The duty ratio determination processing is automatically executed at a timing, e.g., in adjustment immediately before shipment from the factory, in powering the image reading apparatus of the first embodiment, before reading the first original in reading a plurality of originals, or every predetermined time.
Referring back to Fig. 3, the CPU 103 writes the determined duty ratio in the register 128 in step S7. Accordingly, the flickering duty ratio of the light source in a read which starts from a next sync signal SYNC is reserved.
The light source colour and flickering duty ratio for a next line are reserved instead of the light source colour and flickering duty ratio for the current line which is being read in synchronism with the sync signal SYNC, because if the interrupt response time is long, light sources of two colours emit light in one line sync period to degrade the colour reproducibility. In a system in which the interrupt response time is limited to an allowable range, the colour register 125 and PWM waveform generator 127 may be directly connected to the internal bus 124, and the CPU 103 may set colour information in the colour register 125 in step S2 and write the flickering duty ratio in the internal register of the PWM waveform generator 127 in step S7.
As described above, in the first embodiment, the light emission duty ratio of the light sources is set for each color in a read with R, G, and B in accordance with the flow chart shown in Fig. 4. Even if the light sources (LEDs) of the respective colours have different light emission quantities, they can have the same resolution in the subscanning direction. Even when the LED light sources vary in light quantity, they can have the same resolution in the subscanning direction in reading an image by the LED light sources. Therefore, a low-cost reading system with good colour reproducibility of a thin horizontal line or halftone dot image on an original can be realized.
In the first embodiment, an image is read based on the light reflected by an original. However, the present invention is not limited to this, and can be applied to an apparatus of reading an image by, e.g., transmission light from a developed negative film or the like.

Second Embodiment

In the first embodiment, the light sources of the respective colours uniformly flicker during one line sync period, and an output voltage from the image sensor 119 is controlled by the ratio of flickering ON and OFF times, thereby improving the colour reproducibility while preventing an output voltage from the image sensor 119 from exceeding the convertible range of the A/D converter 121.
The R, G, and B read ranges shown in Fig. 2 are wider than those shown in Fig.8. Since the read range is ideally equal to one line width, the first embodiment shown in Fig. 2 exhibits better colour reproducibility than in the prior art shown in Fig. 8 but is poorer in resolution. In the following second embodiment, therefore, the resolution in the subscanning direction is increased while maintaining good colour reproducibility.
Fig. 5 best shows the feature of a colour scanner according to the second embodiment. The same reference numerals as in Fig. 1 denote the same parts, and only a difference from the first embodiment shown in Fig. 1 will be described. As shown in Fig. 5, the second embodiment adopts a timer 133 in addition to the first embodiment.
The input of the timer 133 is connected to the internal bus 124 and the timing control unit 129. When the timer 133 receives a sync signal SYNC from the timing control unit 129, it changes the output to high level and then to low level upon the lapse of a predetermined time stored in an internal register. The output of the timer 133 is connected to switches 111, 112, and 113 in a sensor unit 101 via AND gates 130, 131, and 132.
By this connection, any one of the AND gates 130, 131, and 132 selected by a light source colour designation signal in a colour register 125 starts outputting a waveform generated in a PWM waveform generator 127 in synchronism with the sync signal SYNC, and stops outputting the waveform after the lapse of a predetermined time set in the timer 133. Then, one of LED light sources 108, 109, and 110 starts flickering with a duty ratio set in the PWM waveform generator 127, and stops flickering after the lapse of the predetermined time.
As described above, the input of the timer 133 is connected to the internal bus 124 to allow a CPU 103 to set the predetermined time in the internal register of the timer 133. The CPU 103 sets the time for turning off the LED light sources 108, 109, and 110 in the timer 133 prior to an original read operation.
Parameter setting in the second embodiment will be explained with reference to the flow chart in Fig.7 by exemplifying the case in which one line sync period is 5 ms.
In the following parameter setting operation according to the second embodiment, the total ON time of each LED light source calculated by the CPU 103 is 60% for the R LED light source 108, 30% for the G LED light source 109, and 20% for the B LED light source 110 with respect to one line sync period.
The total ON time of each LED light source is calculated from the product of the duty ratio and one line sync period. In the second embodiment, the CPU 103 calculates the total ON time of each LED light source in one line sync period at a predetermined timing (e.g. , a duty ratio determination timing) by multiplying a duty ratio optimum for the LED light source of each color currently stored in a register 126, i.e., a duty ratio set by the CPU 103 as an optimum duty ratio in the previous parameter setting operation by one line sync period.
In initialization before reading an original, the CPU 103 sets the ON time of a colour having the longest total ON time in the timer 133. In this case, since the R LED light source 108 has the longest total ON time, the CPU 103 sets 5 ms X 60% = 3 ms in the timer 133 . When the timing control unit 129 outputs a sync signal SYNC after the start of read operation, the CPU 103 receives the sync signal SYNC and performs interrupt processing shown in the flow chart of Fig. 7. The interrupt processing has the following steps.
The CPU 103 determines the light source colour for the next line in step S11, and writes the determined colour information in the register 126 in step S12. As a result, the light source colour for a read which starts from the next sync signal SYNC is reserved. The CPU 103 determines the colour for the next output line in step S13. If the colour for the next output line is R, the CPU 103 advances to step S14; if the colour is G, to step S15; and if the colour is.B, to step S16.
The CPU 103 determines the PWM duty ratio in steps S14, S15, and S16 by the same procedure as described in the first embodiment with reference to Fig. 4, and writes the determined duty in a register 128 in step S17. As for R having the longest total ON time, the CPU 103 sets the PWM duty ratio to 100%. As for G, the CPU 103 calculates $\begin{array}{l} Duty Ratio of G \\ = Total ON Time of G / Total ON Time of R \\ = 30 / 60 = 1 / 2 = 50 % \end{array}$
in order to generate a light quantity necessary for G within the same time (3 ms) as the ON time of R. As for B as well as G, the CPU 103 calculates $\begin{matrix} Duty Ratio of B \\ = Total ON Time of G / Total ON Time of R \\ = 20 / 60 = 1 / 3 ≒ 33 % \end{matrix}$
Fig. 6 is a view showing an example in which time which is two-third of the line sync period is set as a predetermined time (the time until the LED light sources 108, 109, and 110 are turned off) counted by the timer 133 by the above method, and the PWM duty ratios of R, G, and B are respectively set to 100%, 50%, and 33%. Since the second embodiment employs this setting, even if the light sources of the respective colours have different light emission quantities, they can have the same resolution in the subscanning direction. Further, the resolution in the subscanning direction can be maximized to the limit of each sensor.
In the second embodiment, the PWM duty ratio is set to 100% for a colour having the longest total ON time, and the duty ratio for another colour is calculated on the basis of the longest total ON time of the colour. The actual original read ranges for the respective colours can be made equal while ensuring necessary light quantities. Consequently, the resolution of a read image can be increased while maintaining good colour reproducibility of a black thin horizontal line or halftone dot image.
In the second embodiment, the R LED light source 108 having the longest ON time has a duty ratio of 100%. In this case, variations in light quantity of the LED light source may influence the resolution in the subscanning direction. For example, when the light quantity of the LED is large, the ON time is controlled short to increase the resolution. To prevent the influence of variations in light quantity of LED on the resolution of a read image, an ON time necessary for an LED having the smallest light quantity, i.e., an LED most greatly varying in light quantity is set as a predetermined time (the time until the LED light source is turned off) of the timer 133. An LED having the smallest light quantity is determined in design with reference to the product specifications (numerical data) of LEDs of the respective colours.
A method of determining the flickering duty ratio of each colour will be described. For example, when the predetermined time, i.e., the time set in design as the time until an LED having the smallest light quantity is turned off is 80% of one line sync period, and the total ON time of a given color by flickering is 60% of one line sync period, the CPU 103 determines the duty ratio of this colour to 60/80 = ¾. By setting (necessary ON time)/(ON time necessary for an LED having the smallest light quantity) as a duty ratio, the resolutions of the respective colours in the subscanning direction can be made equal and increased without any influence of variations in light quantity of the LED. In other words, the resolution can be increased while suppressing variations between LED products and maintaining good colour reproducibility of a black thin horizontal line or halftone dot image.
The present invention may be applied to a system constituted by a plurality of devices (e.g. a host computer, an interface device, a reader, a printer, a scanner, and a facsimile apparatus) or an apparatus comprising a single device (e.g., a copying machine, a scanner, or a facsimile apparatus).
The present invention is realized even by supplying software program codes for realizing the functions of the above embodiments to a computer in a system or apparatus connected to various devices so as to operate these devices in order to realize the functions of the above-described embodiments, and operating the devices in accordance with the program stored in the computer (CPU or MPU) of the system or apparatus. The present invention may be constituted by hardware.
In this case, the software program codes realize the functions of the above-described embodiments by themselves, and the program codes and a means for supplying the program codes to the computer, e.g., a storage medium storing the program codes constitute the present invention. As a storage medium for supplying the program codes, a floppy disk, a hard disk, an optical disk, a magnetooptical disk, a CD-ROM, a magnetic tape, a nonvolatile memory card, a ROM, or the like can be used.
The functions of the above-described embodiments are realized not only when the supplied program codes are executed by the computer but also when the program codes cooperate with the OS (Operating System) running on the computer, another application software, or the like. These program codes are included in the embodiments of the present invention.
The functions of the above-described embodiments are also realized when the supplied program codes are stored in the memory of a function expansion board of a computer or a function expansion unit connected to a computer, and the CPU of the function expansion board or function expansion unit performs part or all of actual processing on the basis of the instructions of the program codes.
As has been described above, according to the first embodiment, the light sources of the respective colours flicker during one accumulation period of the image sensor, and the flickering duty ratio is set for each colour. Even if the light sources of the respective colours have different light emission quantities, they can have the same resolution in the subscanning direction. While suppressing variations in light quantities of the light sources (making the original read ranges of the respective light source colours equal), the colour reproducibility of a thin horizontal line or halftone dot image can be improved.
According to the second embodiment, a light source having the longest ON time in the total ON time of a plurality of light sources during one accumulation period is flickered for a predetermined time during one accumulation period. The ON duty ratio of another light source is set based on the predetermined time and the total ON time set in advance for this light source. As a result, the original read ranges of the respective colours can be made equal and decreased. While maintaining good colour reproducibility of a black horizontal line or halftone dot image, the resolution can be increased.
As many apparently widely different embodiments of the present invention can be made without departing from the scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

Claims

A method of controlling an image reading apparatus which reads an original image by using a plurality of emission colours of light which are sequentially generated by plural light sources in accordance with an emission control signal and detecting light of each colour from the original image by an image sensor, which method includes a step of:
supplying the emission control signal so as to change the emission colour of light generated by said plural light sources (108,109,110) every predetermined charge accumulation period of the image sensor; and is characterised by a step of

turning on-and-off each of all, or all but one, of said plural light sources, a plural number of times uniformly during the charge accumulation period in accordance with a respective duty ratio set for each of the respective emission colours.
The method according to claim 1, which method includes setting a duty ratio at a time when a reference white background is irradiated by turning on-and-off said light source with one of the plurality of emission colours, and an output value from the image sensor is smaller than a predetermined value and comes nearest to the predetermined value as an optimum duty ratio for that emission colour and repeating the duty ratio setting operation for each of the plurality of emission colours.
An image reading apparatus which is adapted to read an original image by using a plurality of emission colours of light which are sequentially generated by plural light sources in accordance with an emission control signal and detecting light of each colour from the original image by an image sensor, which apparatus comprises:
a controller (102,103) adapted to supply the emission control signal so as to change the emission colour of light generated by said plural light sources every predetermined charge accumulation period of the image sensor, characterised in that the said controller is further adapted to turn on-and-off each of all, or all but one, of said plural light sources a plural number of times uniformly during the charge accumulation period in accordance with a duty ratio set for each of the respective emission colours.
The apparatus according to claim 1, wherein said controller is adapted to set a duty ratio at a time when a reference white background is irradiated with one of the plurality of emission colours by turning on-and-off a respective one of said light sources and an output value from the image sensor is smaller than a predetermined value and comes nearest to the predetermined value as an optimum duty ratio for that emission colour, and to repeat this duty ratio setting operation for each of the plurality of emission colours of the other ones of said light sources.
The apparatus according to either one of the preceding claims 3 or 4, wherein said controller is adapted to read an entire original image repeatedly by turning on-and-off said light sources and changing the emission colour at a predetermined pitch a plurality of number of times.
The apparatus according to any one of the preceding claims 3-5, further comprising respective light sources (108,109 & 110) for generating the plurality of emission colours.